Tuesday, February 12, 2013

We All Descend From Winning Guys But Not From Winning Gals

I've spent an interesting hour or two chasing for one particular quote on Roy Baumeister's ode-to-men, also known as his speech about what so stinks in women, and the quote is this one:

The first big, basic difference has to do with what I
consider to be the most underappreciated fact about gender. Consider
this question: What percent of our ancestors were women?

It’s
not a trick question, and it’s not 50%. True, about half the people who
ever lived were women, but that’s not the question. We’re asking about
all the people who ever lived who have a descendant
living today. Or, put another way, yes,every baby has both a mother and a
father, but some of those parents had multiple children.

Recent research using DNA analysis answered this question about two years ago.

I think this
difference is the single most underappreciated fact about gender. To
get that kind of difference, you had to have something like, throughout
the entire history of the human race, maybe 80% of women but only 40% of
men reproduced.

Right
now our field is having a lively debate about how much behavior can be
explained by evolutionary theory. But if evolution explains anything at
all, it explains things related to reproduction, because reproduction is
at the heart of natural selection. Basically, the traits that were
most effective for reproduction would be at the center of evolutionary
psychology.It would be shocking if these vastly different reproductive
odds for men and women failed to produce some personality differences.

Guess what this means, in our Roy's mind? That only the successful men were able to breed, whereas almost all women, however poorly equipped, got to breed. The corollary to this is that men are smarter, more adventurous, better at three-dimensional mental rotation and in general the lords of the creation.

Of course that argument ignores the question why these characteristics would not have been inherited by the daughters of those successful men and not just their sons. Our lack of the required genetic knowledge means that Baumeister wouldn't be able to answer that question. Or rather, that the characteristics of intelligence etc would be passed from father to son alone and the characteristics of mediocrity from mother to daughter alone deserves a little bit more than silence about the possible genetic paths for that

Never mind that one. I looked for the reference to twice as many foremothers as forefathers, and found it after some work. It plunged me into evolutionary molecular biology in which I have the skills of a baby, but the study Baumeister probably used as his source is this:Genetic Evidence for Unequal Effective Population Sizes of Human Females and Males by Jason A. Wilder, Zahra Mobasher, and Michael F. Hammer, 2004. The study is based on genetic data from 73 unrelated men: 25 African Khoisan, 24 Mongolian Khalks and 24 Papua New Guineans. These three samples are viewed as coming from three separate populations.
The introduction to the study states:

Our knowledge of patterns of genetic variation in the human genome is disproportionately shaped by two loci, the mitochondrial DNA (mtDNA) and the nonrecombining portion of the Y chromosome (NRY). Despite this, relatively few studies have directly compared patterns of DNA sequence variation between these two genomic compartments within human populations. In part, this is because the human NRY has extraordinarily low levels of sequence diversity (Malaspina et al. 1990; Dorit, Akashi, and Gilbert 1995; Hammer 1995; Whitfield, Sulston, and Goodfellow 1995; Jaruzelska, Zietkiewicz, and Labuda 1999; Shen et al. 2000; Sachidanandam et al. 2001), which has made characterization of variation difficult and labor intensive, even at global scales (Underhill et al. 1997). In contrast, mtDNA has proved to be a prolific source of DNA variation, even among very local populations (e.g., Vigilant et al. 1991). The primary cause for these disparate patterns is variation in the spontaneous mutation rate, with base substitutions in mtDNA accumulating approximately an order of magnitude faster than in the NRY (Ingman et al. 2000; Thomson et al. 2000). However, once this difference in mutation rate is taken into account, mtDNA and the NRY should reveal similar evolutionary histories, assuming they are evolving neutrally in a panmictic population with an equal breeding sex ratio. The degree to which these conditions are satisfied, and the extent to which evolutionary forces equally influence mtDNA and the NRY, remain open questions in human evolutionary genetics.

One of the most intriguing observations regarding the evolutionary histories of human mtDNA and Y chromosomes is that they are estimated to have very different times to the most recent common ancestor (TMRCA), with that of mtDNA estimated at 171.5 to 238 thousand years ago (kya) Ingman et al. 2000; Tang et al. 2002) and estimates for the NRY ranging between 46 and 109 kya in recent studies (Pritchard et al. 1999; Thomson et al. 2000; Hammer and Zegura 2002; Tang et al. 2002). Because the TMRCA of a selectively neutral locus is influenced primarily by its effective population size (Ne), the observed disparity between mtDNA and the NRY is somewhat unexpected. These loci are typically assumed to have equal Ne values in neutral evolutionary models and are, therefore, also expected to have similar TMRCAs. Taken alone, however, it is difficult to determine whether the observed difference between mtDNA and the NRY reflects anything more than simple stochasticity in the coalescent process (e.g., Hudson and Turelli 2003). Multilocus comparisons with other portions of the genome, however, indicate that the NRY has significantly less diversity (and, thus, a shorter genealogy) than expected under a standard neutral model (Shen et al. 2000). Although the reasons for this reduction in variation remain unclear, these findings suggest that mtDNA and the NRY may be influenced differently by natural selection or sex-specific demographic processes.

Got it? The idea is to employ various models, assumptions and statistical programs to try to figure out how far back one needs to go to find a common male ancestor in each of the three studied populations (by using the NRY) and how far back one needs to go to find a common female ancestor in each of the three studied populations.

I'm not qualified to discuss whether those processes an assumptions are valid, so keep that in mind. What the study found was that the time to the most recent common male ancestor was roughly half of that to the most recent common female ancestor.

This is the finding which Baumeister translates as the effects of more intense sexual selection on men than on women and the assumption that twice as many women as men have passed their genes on. Because that shared grandpa is so much more recent than that shared grandma, humans have more grandmas than grandpas and the grandpas were the winners. Rather than being the winners of the genetic race, us women are instead viewed as not having evolved very much at all. Sniff.

And it could be the case that more intense sexual competition meant that fewer men than women passed their genes on, especially if those ancient tribal units practiced some amount of polygyny (one man with more than one woman) though that doesn't prove, in itself, that the men who succeeded in passing their genes on were all those wonderful things Baumeister dreams about. But there are, in fact, alternative theories for the findings.

It may not be sexual selection which caused that difference (assuming the difference remains verified in future studies) but simply natural selection:

A leading hypothesis to explain the comparatively recent ancestry of the human NRY is that positive directional selection has played a strong role in shaping nucleotide diversity in this compartment of the genome (Malaspina et al. 1990; Dorit, Akashi, and Gilbert 1995; Whitfield, Sulston, and Goodfellow 1995; Jaruzelska, Zietkiewicz, and Labuda 1999; Pritchard et al. 1999). Because it is non- recombining and haploid, the NRY acts as a single locus that may be particularly prone to periodic diversity-reducing selective sweeps (e.g., Maynard Smith and Haigh 1974; Begun and Aquadro 1992).

Luca Cavalli Sforza and Marcus Feldman proposed that explanation in 2003 (The Application of Molecular Genetic Approaches to Human Evolution)

Or it could be stochastic, as Rosalind Harding and Gil McVean proposed in 2003 (A structured ancestral population for the evolution of modern humans)

Other theories I came across (assuming they are not just sub-examples of those already cited) include the longer generation length of men and the idea that men may have suffered from higher mortality (perhaps in early childhood?) than women so that the sizes of the breeding populations might not have been equal for the two sexes. In short, there may have been fewer fertile men than fertile women, in which case the time to the most recent common ancestor (TMRCA) would be miscalculated.

Finally, the effect could also be caused by patrilocal "marriage" customs which have been the most common form. This means that it is women who move when entering "marriage" (or whatever long-term mate contracts were called), not men. Over time the population will show much more variety in the mitochondrial DNA (which is inherited solely from the mother) than in NRY (which men inherit only from their fathers) because the population builds itself on the basis of related males and unrelated females who came from outside the group. This differential variety would produce the same apparent time to the most recent common ancestor results.

I hope I got some of that right! The point is that a statement I recently saw somewhere stating that it is definitely proved that twice as many women as men have passed their genes on (DNA!) isn't quite so definite inside evolutionary molecular biology than it is inside the evolutionary psychology amateurs' understanding.

It's hard for me to know if the studies I found give the most recent views. But that's not really what my task here has been. It has been to show that it always pays to ask how someone knows what they say they know. Always verify!

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